The invention relates to rotating electric machinery; it relates more particularly to electric motors and generators in which radiative heat exchange between the opposite surfaces of the stator and rotor assemblies is enhanced by means of surface coatings.
The designers of rotating electric machinery have been traditionally aware of the substantial role that heat dissipation plays in the operation of electric motors and generators since the permissible temperature rise of the materials utilized to insulate the conductors in these components sets a practical limit to the reduction in size and improvement in efficiency of their products. In particular, the dissipation of the heat generated within the windings of the rotor assembly has been a constant concern due to the effective insulation, in the thermal sense, of that component from the heat sink represented by the atmosphere surrounding the motor. In the discussion below of the principles of the invention, such devices will be commonly referred to as motors; the term is intended to include analogous machines employed as generators, stepping motors, rotating relays, and their like.
The principal reliance of the prior art has been on the provision of means for improving convective heat transfer from the rotor, generally by means of forced gas flow in the airgap between the rotor and the stator. Atmospheric air is generally employed as the heat transfer medium, and a fan mounted coaxially with the rotor on the motor shaft as the means for forcing the fluid through the airgap.
The reliance of the prior art on convective heat transfer has resulted in manifold problems, because the need to provide relatively large airgaps for coolant passage conflicts with the need to reduce the dimension of that airgap so as to increase the magnetic coupling efficiency between the stationary and rotating magnetic fields of the machine. To compromise such conflicting requirements in certain high-grade applications, such as large generators, the prior art has employed a fluid of relatively low viscosity and high specific heat--typically helium or hydrogen--to act as the coolant, with a secondary heat exchanger for the transfer of the removed heat to the atmosphere.
Such methods and techniques of the prior art are obviated and improved in the electric motor of the invention, wherein heat transfer between the rotor and the stator is principally by means of radiative exchange, such exchange being enhanced by the provision of suitable coatings on the opposing surfaces of these components. Since radiative heat transfer is insensitive to the distance between the emitter and the receptor, and may be improved by the reduction in the mass of air in the radiation path, the method of the invention is fully compatible with an improvement in the magnetic coupling between the stator and the rotor.